1. Field of the Invention
The present invention relates to an analog-to-digital converter, a solid-state imaging device including the same, and a method of digitizing analog signal.
2. Description of the Related Art
A means known so far as a system which digitizes a pixel signal in a CMOS image sensor continues counting until a voltage which is generated by a reference voltage generation circuit and changes in the shape of a slope with time reaches a voltage output from a pixel section to digitize a video signal output from the pixel section. Specifically, a difference between the voltage of the video signal output from the pixel section and the reset voltage which is a reference for this video signal is digitized. However, this technique generates thermal noise upon sampling of the potential difference.
Then, the following technique is known in consideration of the above-mentioned problem. First, after digitization of the reset voltage using the voltage output from the reference voltage generation circuit, the video signal is digitized using the voltage output also from the reference voltage generation circuit. Then, the difference between the two digitization results is adopted as the final digitization quantity, i.e., a digital quantity (see W. Yanget et al. and “An Integrated 800×600 CMOS Imaging System” ISSCC Digest of Technical Papers and February, 1999, pp. 304-305). However, since the difference between the digitization results of two times is adopted as the digital quantity, the above-mentioned technique has a fault that the thermal noise generated in the pixel section, the reference voltage generation circuit, and a comparator which are included in a solid-state imaging device is added twice.